The present invention relates to equalization of transmission channels, and more particularly to a circuit for sensing cable effects to the determine the amount of equalization required for automatic equalization.
Equalization of transmission channels, often simply wire cables, is common in the field of data communication. The desired effect of equalization is to compensate for the high frequency loss of the cable so that the received waveform more closely resembles the transmitted waveform. This equalization reduces phase and amplitude distortions which otherwise can cause transmission errors. Fixed equalization may be used when the cable type and length are constant and known. Adjustable equalization may be used to manually accommodate differing cable lengths. Automatic equalizers determine the required equalization from the characteristics of the received signal, and then vary the applied equalization accordingly.
Various sensing schemes are currently used in automatic equalizers. Some methods require transmission of a training sequence, i.e., a known signal from which the receiver can derive an optimum equalization filter response. Another method senses the amplitude of the received signal and applies equalization to restore the signal to an assumed initial amplitude. Both of these methods put certain requirements on the transmitted signal, either that it contain the training sequence or that it always have an accurately fixed amplitude at launch.
What is needed is a method for sensing the required equalization for automatic equalization without the requirement of training signals or fixed amplitude signals.